The present invention relates to a focus state detection apparatus used in an image sensing apparatus, such as a still camera and a video camera, and various kinds of observation apparatuses and, more particularly, to a focus state detection apparatus which performs focus state detection by using area sensors configured with two-dimensional solid-state image sensing devices, such as CCDs, capable of sensing an complete image.
FIG. 31 is a view illustrating a brief optical configuration of a camera including a conventional focus detection unit. In FIG. 31, reference numeral 101 denotes an object lens which introduces an image of an object (referred as xe2x80x9cobject imagexe2x80x9d, hereinafter) into the apparatus; 102, a main mirror (half mirror) which is half transparent and reflects a part of light of the object image incoming through the object lens 101; 103, a reticle which is placed at a focal plane of the object lens 101; 104, a pentagonal prism which changes the traveling direction of light; 105, an eyepiece; 106, sub-mirror which operates when performing focus state detection; 107, a film, such as a silver halide film; and 108, a focus state detection unit.
Referring to FIG. 31, light from an object (not shown) passes through the object lens 101, then a part of the light is reflected by the main mirror 102 upward, and the reflected light forms an image on the reticle 103. The image formed on the reticle 103 is further reflected in the pentagonal prism 104 a plurality of times, and eventually reaches the eye of a user through the eyepiece 105. Further, the light which passes through the main mirror 102 reaches the film 107 and exposes it with the object image thereby obtaining a desired image.
Meanwhile, a part of the flux of light which passed through the main mirror 102 is reflected by the sub-mirror 106 downward, and led to the focus state detection unit 108.
FIG. 32 is a view for explaining the principle of focus state detection in relation to the object lens 101 and the focus state detection unit 108 shown in FIG. 31.
In the focus state detection unit 108 shown in FIG. 32, reference numeral 109 denotes a field stop provided near the desired focal plane, i.e., a conjugate plane of a plane where the film 107 is supplied; 110, a field lens arranged near the desired focal plane; 111, a secondary lens system having two lenses 111-1 and 111-2; 112, a photoelectric conversion device including two line sensors 112-1 and 112-2 provided behind the lenses 111-1 and 111-2, respectively; 113, an iris diaphragm having two aperture openings 113-1 and 113-2 corresponding to the lenses 111-1 and 111-2 of the secondary lens system 111, respectively; and 114, an exit pupil of the object lens 101. Note, the field lens 110 has power for forming an image of the aperture openings 113-1 and 113-2 of the iris diaphragm 113 in near areas 114-1 and 114-2 of the exit pupil 114 of the object lens 101. In reverse, fluxes of light 115-1 and 115-2 which passed through the areas 114-1 and 114-2 further pass through the aperture openings 113-1 and 113-2, respectively, and incident on the two line sensors 112-1 and 112-2, thereby distributions of quantity of light are obtained by the two line sensors 112-1 and 112-2.
The focus state detection unit 108 shown in FIG. 32 adopts a so-called phase-difference detection method. When the focal point of the object lens 101 is in front of the desired focal plane, namely when an image is focused ahead of the desired focal plane, the images obtained by the two line sensors 112-1 and 112-2 approaches each other. In opposite, when the focal point of the object lens 101 is behind the desired focal plane, the images obtained by the two line sensors 112-1 and 112-2 recedes from each other. Since the shifted amount between the distributions of quantity of light of the two line sensors 112-1 and 112-2 has a predetermined functional relationship to a defocus amount of the object lens 101, by calculating the shifted amount between the distributions in accordance with proper operation, it is possible to obtain defocus direction and amount. The object lens 101 is moved in accordance with the defocus direction and amount so that the defocus amount approaches 0. When the defocus amount becomes substantially 0, the focus state detection is finished.
In the camera including the conventional focus state detection unit 108 as shown in FIG. 32, an area used for the focus state detection (referred as xe2x80x9cdetection areaxe2x80x9d, hereinafter) is a strip and narrow as an area B with respect to an sensed image area A as shown in FIG. 33. The size and shape of the detection area B is determined by the shape of the line sensors 112-1 and 112-2, shown in FIG. 32, used in the focus state detection.
FIG. 34 is a block diagram showing a brief mechanism for charge control of the line sensors 112-1 and 112-2. Referring to FIG. 34, an output VD, commonly used as a reference for the line sensors 112-1 and 112-2, of a light-blocked pixel 120 (the pixel is referred as xe2x80x9cdark pixelxe2x80x9d and the output is referred as xe2x80x9cdark voltagexe2x80x9d, hereinafter), and an output VP from a maximum voltage detection circuit 121 connected to the line sensors 112-1 and 112-2, namely the maximum voltage of the line sensors 112-1 and 112-2, are inputted to a differential amplifier 122. Then, the difference between the dark voltage VD and the maximum voltage VP is obtained and outputted. Charging of the line sensors 112-1 and 112-2 continues until the difference reaches a predetermined level VR, and when the difference reaches the predetermined level VR, charging of the line sensors 112-1 and 112-2 is terminated and a signal xcfx86R which is an end-charging signal for transferring the stored charges from pixels to charge capacitors is sent to the line sensors 112-1 and 112-2. The reason for taking a difference between the maximum voltage VP and the dark voltage VD is that, by charging the line sensors 112-1 and 112-2 until the difference between the maximum voltage VP and the dark voltage VD reaches the predetermined level VR, it is possible to obtain the phase difference between the distributions of quantity of light for focus state detection in sufficient precision. Further, if charging is continued after the difference reaches the predetermined level, there is a possibility that the pixels of the line sensors 112-1 and 112-2 saturate, which may cause improper focus state detection. Therefore, when xe2x80x9cVPxe2x88x92VD=VRxe2x80x9d is satisfied, the end-charging signal xcfx86R is outputted to the line sensors 112-1 and 112-2.
FIGS. 35A and 35B are graphs showing image signals (distributions of quantity of light) from the line sensors 112-1 and 112-2 with reference to the dark voltage VD of the dark pixel 120, and the maximum voltage VP of first and second images (in FIGS. 35A and 35B, the maximum voltage VP is in the first image), corresponding to the line sensors 112-1 and 112-2, respectively, is the predetermined level VR. For using the signals from the line sensors 112-1 and 112-2 for focus state detection, when the difference between a voltage of any pixel of the line sensors 112-1 and 112-2 and the dark voltage VD reaches the predetermined level VR, charging is terminated and whether or not an image is focused is determined on the basis of output images.
FIG. 36 is a circuit diagram showing a brief configuration of the maximum voltage detection circuit 121 and its subsequent circuits, namely the differential amplifier 122 and a part of a charge controller 123 both shown in FIG. 34. In FIG. 36, only two sets of circuits for outputs Vn and Vnxe2x88x921 outputted from n-th and (nxe2x88x921)-th pixels, respectively, are connected to a wire 136, however, the same number of similar circuits as that of pixels included in the line sensors 112-1 and 112-2 are actually connected to the wire 136. Each pixel output is compared to the current maximum voltage VP, and when an pixel output Vn of the n-th pixel exceeds the current maximum voltage VP, an output from a differential amplifier 130 n is reversed and a MOS switch 132 n turns ON. Accordingly, the pixel output Vn is outputted through a voltage follower 131 n to the wire 136, thereby the pixel output Vn becomes the new maximum voltage VP. The maximum voltage VP of the line sensors 112-1 and 112-2 outputted from the maximum voltage detection circuit 121 enters the differential amplifier 122, where the difference between the maximum voltage VP and the dark output VD is obtained and amplified. Further, the output from the differential amplifier 122 is compared with the predetermined level VR by a comparator 134, and when the output from the differential amplifier 122 exceeds the predetermined level VR, then the end-charging signal xcfx86R is outputted thereby the charging is terminated. Thereafter, a signal xcfx86RESET is applied to a gate 135 to ground the wire 136, thereby resetting the wire 136 for preparing for the next charge control.
FIG. 37 shows an example of expanded detection areas B to be used for the focus state detection. There are three detection areas Bxe2x80x2 used for the focus state detection in the sensed image area A. The detection areas Bxe2x80x2 in FIG. 37 are obtained by adding three more stripe areas which run in the direction perpendicular to the area B shown in FIG. 33.
FIG. 38 shows an example of an arrangement of line sensors corresponding to the areas Bxe2x80x2 shown in FIG. 37. When the areas Bxe2x80x2 are as shown in FIG. 37, a photoelectric conversion element including plural pairs of line sensors C to F (referred as xe2x80x9cline sensor pairsxe2x80x9d, hereinafter), shown in FIG. 38, and a corresponding lens system (not shown) are provided.
Further, as for the charge control of the plural line sensor pairs C to F, peripheral circuits, such as the one shown in FIG. 34, and a plurality of charge controllers 149 to 152 are provided for respective line sensor pairs C to F in order to control by pair, as shown in FIG. 39. Referring to FIG. 39, differential amplifiers 145 to 148 take differences between dark voltages VD1 to VD4 of the line sensor pairs C to F and maximum voltages VP1 to VP4 outputted from maximum voltage detection circuits 141 to 144, respectively, the differences are compared to the predetermined level VR by charge controller 149 to 152. When a difference exceeds the predetermined level VR, corresponding one out of the end-charging signals xcfx86R1 to xcfx86R4 is outputted in order to terminate charging all the pixels included in the corresponding one of the line sensor pairs C to F. After each line sensor pairs finishes charging, charges are outputted from all the pixels of the each line sensor pairs as image signals via a signal line (not shown), then, a defocus amount is detected on the basis of the image signals.
The above is the description of the focus state detection unit using strip sensors, i.e., line sensors. The detection area or areas correspond to photo-reception areas of the line sensors, therefore, the shape of the detection area or areas is limited to a line, a plurality of lines, or a combination thereof.
Accordingly, if further expansion of the detection area is directed, a photoelectric conversion unit having photo-reception areas extending in two dimensions are needed.
FIG. 40 shows a detection area Bxe2x80x3 with respect to the sensed image area A in the focus state detection unit using area sensors. As seen from FIG. 40, the detection area Bxe2x80x3 is extended greatly compared to the areas B and Bxe2x80x2 shown in FIGS. 33 and 37. If the phase-difference detection method is adopted, the photoelectric conversion unit has two two-dimensional photo reception areas, namely, a pair of area sensors (referred as xe2x80x9carea sensor pairxe2x80x9d, hereinafter) 160-1 and 160-2 as shown in FIG. 41. By dividing photo reception areas of the area sensor pair 160-1 and 160-2 into a plurality of areas (called xe2x80x9cdivided areasxe2x80x9d, hereinafter) and detecting a phase difference by each divided area pair, focus state detection can be performed in two dimensional area.
However, there is a problem on relationship between detection precision and the number of pixels. More specifically, when the photo reception areas are divided into many areas for making the best use of the expanded photo reception areas of the area sensors, the number of pixels included in each area decreases, which may result in unsatisfactory precision of detected focus state. In contrast, when the photo reception areas are divided so that each detection area includes the sufficient number of pixels, detection centers (central portions of the respective divided areas) are separated by considerable intervals from each other. In this case, the focus state detection unit may not have good operability since the detection centers are scattered in the photo reception areas.
More specifically, a problem due to the number of each detection area is substantially solved if the divided areas are set so as to obtain an arrangement of detection centers as shown in FIG. 42 (i.e., five detection centers in the vertical direction and nine detection centers in the horizontal direction) in the detection area Bxe2x80x3, namely, in each of area sensors 160-1 and 160-2. However, if the photo reception areas are divided in this manner (five divided areas in the vertical direction, as shown in FIG. 43A), the number of pixels included in each area is small. It is possible to increase the number of pixels by decreasing the size of each pixel, however, another problem for realizing high efficient photoelectric conversion with narrower aperture openings of down-sized pixels arises.
Further, in cases shown in FIG. 43B (three divided areas in the vertical direction) and in FIG. 43C (two divided areas in the vertical direction), sufficient precision in focus state detection can be achieved for each divided area, however, the number of the detection centers is not satisfactory. It is possible to use both of these two types of divisions when performing focus state detection. In such case, in order to obtain optimized image signals in each divided area, charging operation has to be performed twice in each division state.
Regarding charge control of the area sensors 160-1 and 160-2, they are collectively controlled by a maximum voltage detection circuit 161 (see FIG. 44) using the common dark voltage VD, a differential amplifier 162, and a charge controller 163.
For the sake of simple explanation, it is assumed that an image corresponding to an image signal Y, as shown in FIG. 45, used for focus state detection is formed on an area sensor 160-1 or 160-2, and the area sensor 160-1 or 160-2 is divided into four areas, G to J.
FIGS. 46A to 46 D are graphs of image signals obtained in the four areas G to J, respectively, which are shown in FIG. 45. As seen from the graphs, since the area sensors are collectively controlled, charge stored in the area H which includes a pixel charged to the maximum level in the area sensor 160-1 or 160-2 is in ideal level, however, charge stored in the other areas G, I and J are not in ideal levels. In this case, although the detection area is extended, an area to be used for focus state detection is not extended, which wastes an advantage of using area sensors.
Further, in an image sensing apparatus using line sensors, variation in focus state detection precision due to a way an image is formed on the line sensors (so called xe2x80x9cphase-in and phase-outxe2x80x9d problem) has been a problem. This problem remains even when area sensors are used.
Furthermore, the phase-in/phase-out problem is more serious when the object image is formed on the area sensor pair, since the distortion of the object image becomes heavy when it extends in two dimensions. As a specific problem, there is a gap between an image inside of a marked area, showing an image portion expectedly used for the focus state detection, seen from a finder and an image portion actually used for focus state detection. In this case, the image may be focused on an undesired portion which does not include the object. In order to reduce the distortion of the image, optical members for optically correcting the distortion of the image are required. However, it is technically difficult to design such the optical members and the configuration of an apparatus becomes very complicated. Thus, to correct the distortion of an image is not easy.
The present invention has been made in consideration of the above situation, and has as its object to provide a small inexpensive focus state detection apparatus capable of independently controlling each of a plurality of divided areas of area sensors and focusing on an object at an arbitrary position in an image frame at high precision.
According to the present invention, the foregoing object is attained by providing a focus state detection apparatus which detects focus state of an object on the basis of signals obtained from light flux from the object passed through an optical system, the apparatus comprising: a pair of image sensing devices each of which extends in two dimensions and is divided into a plurality of areas each of which is controlled independently; control means for independently controlling charging of each pair of the corresponding divided areas of the pair of image sensing devices; and detection means for detecting focus state on the basis of signals outputted from each pair of divided areas of the pair of image sensing devices after charging of the pair of divided areas is finished under control of the control means.
Further, it is another object of the present invention to provide a focus state detection apparatus, which performs focus state detection on any position in a continuous two-dimensional sensed or observation image, capable of improving unevenness in detected result of focus state, caused by distortion of an object image on the photoelectric conversion devices (so-called phase-in/phase-out problem).
According to the present invention, the foregoing object is attained by providing a focus state detection apparatus which detects focus state of an object on the basis of signals obtained from light flux from the object passed through an optical system, the apparatus comprising: a pair of image sensing devices each of which extends in two dimensions and is divided into a plurality of areas each of which is controlled independently; control means for independently controlling charging of each pair of the corresponding divided areas of the pair of image sensing devices; and detection means for detecting focus state on the basis of signals outputted from each pair of divided areas of the pair of image sensing devices after charging of the pair of divided areas is finished under control of the control means, wherein the pair of image sensing devices are designed so as to compensate for distortion of an image of the object caused by the optical system.